As the capacity requirements of electrical devices continue to increase, expectations for improving the energy density of lithium batteries are also rising. This is especially true for portable devices such as smartphones, tablets, and laptops, which demand smaller size and longer standby time from lithium batteries. Similarly, other electrical devices, such as energy storage devices, power tools, and electric vehicles, are constantly developing lithium batteries that are lighter, smaller, and have higher output voltage and power density. Therefore, developing high-energy-density lithium batteries is a crucial research and development direction for the lithium battery industry.
I. Background of High-Voltage Lithium-ion Battery Development
In order to design high-energy-density lithium batteries, in addition to continuously optimizing their space utilization, improving the compaction density and specific capacity of the positive and negative electrode materials, and using highly conductive carbon nanotubes and polymer binders to increase the content of positive and negative electrode active materials, increasing the operating voltage of lithium batteries is also one of the important ways to increase battery energy density.
The cutoff voltage of lithium batteries is gradually transitioning from the original 4.2V to 4.35V, 4.4V, 4.45V, 4.5V, and 5V. Among them, the 5V nickel-manganese lithium battery has excellent characteristics such as high energy density and high power, and will be one of the important directions for the future development of new energy vehicles and energy storage. With the continuous development of power supply R&D technology, higher voltage and higher energy density lithium batteries will gradually move out of the laboratory and serve consumers.
II. Current Status of High-Voltage Lithium-ion Battery Applications
High-voltage lithium batteries generally refer to batteries with a single-cell charging cutoff voltage higher than 4.2V, such as those used in mobile phones. The cutoff voltage has evolved from 4.2V to 4.3V, 4.35V, and then to 4.4V (Xiaomi phones, Huawei phones, etc.). Currently, 4.35V and 4.4V lithium batteries are maturely used in the market, while 4.45V and 4.5V batteries are beginning to gain market favor and will gradually mature.
Currently, both domestic and international manufacturers of batteries for mobile phones and other digital electronic products are moving towards high-voltage lithium batteries. High-voltage and high-energy-density lithium batteries will have a larger market share in high-end mobile phones and portable electronic devices. Cathode materials and electrolytes are key materials for improving the high voltage of lithium batteries, with modified high-voltage lithium cobalt oxide and high-voltage ternary materials becoming more mature and widespread.
High-voltage lithium batteries experience a decrease in certain safety features during use as the voltage increases, thus limiting their widespread adoption in electric vehicles. Currently, the primary cathode materials used in electric vehicle batteries are ternary lithium batteries and lithium iron phosphate batteries. To improve energy density and meet demand, high-nickel cathode materials such as 811NCM and NCA, high-capacity silicon-carbon anodes, or methods to increase battery space utilization are generally employed to enhance energy density and driving range.
III. Current Status of Key Materials and Processes for High-Voltage Lithium Batteries
The performance of high-voltage lithium batteries is largely determined by the structure and properties of the active materials and electrolytes, with the cathode material being the most critical core material, and the matching application of the electrolyte also being extremely important. The following is an important analysis of the current research and application status of high-voltage cathode materials.
1. Current Status of Research on High-Pressure Lithium Cobalt Oxide Materials
The most widely researched and applied high-voltage cathode material is lithium cobalt oxide, which has a two-dimensional layered structure, α-NaFeO2 type, making it more suitable for lithium-ion insertion and extraction. Lithium cobalt oxide has a theoretical energy density of 274 mAh/g and boasts advantages such as simple production process and stable electrochemical properties, resulting in a high market share. In practical applications, only a portion of lithium ions in lithium cobalt oxide can reversibly insert and extract, with an actual energy density of approximately 167 mAh/g (operating voltage 4.35V). Increasing its operating voltage can significantly improve its energy density; for example, increasing the operating voltage from 4.2V to 4.35V can increase the energy density by about 16%. However, under high voltage, repeated insertion and extraction of lithium ions from the material causes the structure of lithium cobalt oxide to transform from a trigonal to a monoclinic crystal system. At this point, the lithium cobalt oxide material no longer has the ability to insert and extract lithium ions, and the cathode material particles loosen and detach from the current collector, leading to increased internal resistance and deteriorated electrochemical performance.
Currently, the modification of lithium cobalt oxide cathode materials mainly focuses on improving the crystal structure stability and interface stability of the materials through two aspects: doping and coating.
Currently, high-voltage lithium cobalt oxide materials are being used in large quantities in high-energy-density batteries. For example, high-end mobile phone battery manufacturers have increasingly higher requirements for battery performance, particularly in terms of energy density. A key requirement is around 660Wh/L for a 4.35V mobile phone battery using carbon as the negative electrode, while a 4.4V battery has reached around 740Wh/L. This necessitates higher compaction density, higher void space utilization, and better structural stability of the positive electrode material under high compaction and high voltage conditions. However, lithium cobalt oxide electrode materials suffer from drawbacks such as the scarcity and high cost of cobalt resources. Furthermore, cobalt ions possess a certain degree of toxicity, which limits their widespread application in power lithium batteries.
2. Current Status of Ternary Materials Research
To reduce cobalt usage and improve battery safety, researchers have begun focusing on layered ternary high-voltage materials (LiNixCoyMn1-x-yO2 or LiNixCoyAl1-x-yO2). In these ternary materials, nickel (Ni) provides capacity, cobalt (Co) reduces the mixing of lithium (Li) and Ni, and manganese (Mn) or aluminum (Al) improves the structural stability of the layered material, thereby enhancing battery safety. These batteries are primarily used in general digital batteries, such as power banks and commercial backup batteries, serving as a substitute for lithium cobalt oxide to improve price competitiveness. A nickel-cobalt-manganese ratio of 5:2:3 is most common. Many manufacturers are experimenting with these in the automotive sector, primarily by increasing the operating voltage of individual lithium-ion cells and adding nickel content to the ternary materials. However, the industry is still in the development stage, and mass-produced products are not yet available. The key is that current power lithium-ion batteries must prioritize high safety, consistency, low cost, and long lifespan; increasing capacity is not the primary concern.
A significant problem with ternary materials is that as the nickel content increases, the material becomes more alkaline, placing increasingly stringent demands on battery manufacturing processes and environmental conditions. Simultaneously, the material's thermal stability decreases, and it releases oxygen during cycling, leading to poorer structural stability. Furthermore, nickel's strong oxidizing properties during charging place even greater demands on electrolyte compatibility. Therefore, ternary electrode materials face considerable limitations in their widespread adoption and application.
3. Current Status of Research on Manganese-Based Cathode Materials
Lithium manganese oxide is a typical spinel-type cathode material with a theoretical energy density of 148 mAh/g reported in the literature. Its energy density is lower than that of lithium cobalt oxide and ternary materials. It has the advantages of low price, high thermal stability, environmental friendliness and easy preparation, and is expected to be widely used in energy storage batteries and power lithium batteries.
In the application of lithium manganese oxide in power lithium batteries, its use in China is not as widespread as that of ternary materials and lithium iron phosphate, mainly due to its low energy density and poor cycle life, resulting in short driving range and short lifespan. The cycle performance of lithium manganese oxide, especially its high-temperature (55℃) cycle performance, has been widely criticized. The main influencing factors are threefold: ① Dissolution of surface Mn3+. Since the lithium salt used in conventional electrolytes is lithium hexafluorophosphate (LiPF6), the electrolyte itself contains a certain amount of hydrofluoric acid (HF) impurities. Trace amounts of water in the battery system can cause the decomposition of LiPF6, producing HF. The presence of HF corrodes lithium manganese oxide (LiMn2O4) and causes Mn3+ to disproportionately dissolve: 2Mn3+ (solid phase) → Mn4+ (solid phase) + Mn2+ (solution phase). At the end of discharge and under high-rate discharge conditions, the Mn3+ content on the material surface is higher than that in the bulk phase, exacerbating the dissolution of Mn3+ on the material surface. ② The Jan Taylor effect. During battery discharge, especially under over-discharge conditions, the Li1+δ[Mn2]O4 formed on the material surface is thermodynamically unstable. Simultaneously, the material structure undergoes a transformation from cubic to tetragonal phase, disrupting the original structure and thus deteriorating the material's cycle performance. ③ High oxidizing power of Mn4+. At the end of charging or under overcharge conditions, Mn4+ in the highly delithilated Li1+δ[Mn2]O4 material exhibits strong oxidizing power.
Oxidizing properties can oxidize and decompose organic electrolytes, deteriorating battery cycle performance. Currently, most lithium manganese oxide batteries have an energy density of less than 100 mAh/g, achieving only 400-500 cycles at room temperature and only 100-200 cycles at high temperatures, which cannot meet mass production requirements. However, the Nissan Leaf, which accounts for nearly 20% of global electric vehicle sales, uses lithium manganese oxide batteries, achieving a range of approximately 200 km.
Although the performance of lithium manganese oxide batteries is limited by the material's structure, as long as its shortcomings of low energy density and poor cycle performance are addressed, it still has a very broad application prospect in the field of power lithium batteries in the future.
To improve the energy density and cycle performance of lithium manganese oxide electrode materials, some researchers have increased the voltage of cathode materials through doping modification, such as the 5V high-voltage cathode material LiMxMn2-xO4 [(M=chromium(Cr), iron(Fe), Co, Ni, copper(Cu)], among which the nickel-manganese high-voltage material LiNi0.5Mn1.5O4 has been the most extensively studied. The nickel-manganese high-voltage material has a discharge specific capacity as high as 130 mAh/g, a plateau of approximately 4.7V, and an energy density higher than that of cobalt oxide at conventional operating voltages. Lithium has a high energy density and virtually no Jameer-Taylor effect like Mn3+. When the operating voltage is increased to around 5V, nickel-manganese high-voltage materials, compared with traditional lithium cobalt oxide, lithium manganese oxide, ternary lithium, and lithium iron phosphate, have advantages such as high specific capacity, high discharge platform, and high safety and rate performance. They have significant advantages in battery pack assembly, but their high-temperature performance and cycle life need improvement. Currently, their application is limited to small-batch production of steel-cased batteries, and there is still a long way to go in the doping modification and surface coating of nickel-manganese high-voltage materials.
4. Current Status of Research on High-Voltage Electrolytes
While high-voltage lithium batteries have made significant contributions to improving battery energy density, they also present numerous challenges. As energy density increases, the compaction density of both the positive and negative electrodes typically becomes higher, leading to poorer electrolyte wettability and reduced electrolyte retention. Low electrolyte retention results in deteriorated battery cycle and storage performance. In recent years, with the emergence and application of high-voltage cathode materials, conventional carbonate and lithium hexafluorophosphate systems have decomposed in batteries above 4.5V, exhibiting poor cycle performance, poor high-temperature performance, and other performance degradations, thus failing to fully meet the requirements of high-voltage lithium batteries. Therefore, researching electrolyte systems that are well-matched to these high-voltage cathode materials is of paramount importance.
To address the poor electrolyte wettability caused by high compaction density, electrolyte design teams are continuously screening solvents with high oxidation potential and low viscosity to meet the performance requirements of high compaction batteries. Additionally, additives or fluorinated solvents that improve electrolyte wettability are being used, with noticeable results.
